1. Field of the Invention
The present invention relates to magnetostrictive position sensing apparatus, and more particularly to the use of such apparatus to sense the position of movable parts in digitally controlled apparatus such as photographic apparatus.
2. Description of the Problem
Many modern photographic cameras are highly automated, including automatic features such as exposure control, focus, flash operation, film advance, etc. Increasingly, as the cost of integrated circuitry declines, the method of choice for controlling such highly automated photographic cameras is the use of digital logic. U.S. Pat. No. 4,103,307 issued July 25, 1978 to Shimoda et al, represents an example of a single lens reflex camera with automatic exposure control, that is controlled by a digital computer.
One of the most difficult problems faced in the design of such a camera is how to get information regarding the positions of various camera parts such as the lens, shutter, aperture, or reflex mirror, and the positions of numerous operator controlled knobs, buttons and dials into the computer.
In the past, prior to the widespread use of digital circuitry, potentiometers were widely employed as transducers for indicating to the camera control circuitry the position of moving parts. Switches were also employed when a binary (on or off) type indication was needed. See for example, U.S. Pat. No. 3,836,920 issued Sept. 17, 1974 to Uchiyama et al.
A problem was encountered, however, with the use of potentiometers as position transducers for digital control circuitry. The analog voltage generated by the potentiometers needs to be converted to a digital value by an analog-to-digital converter before the information is useful to the digital computer. The required analog-to-digital conversion circuitry is costly and complicated. Another problem encountered with potentiometers employed as position transducers is their susceptibility to noise, as dirt accumulates between the sliding contacts and the resistive element of the potentiometer. Various forms of transducers using a plurality of contacts were proposed as a cure for these problems. See for example U.S. Pat. No. 4,079,386 issued Mar. 14, 1978 to Murakami et al, wherein a position sensor including a pulse generator comprising a comb-shaped conductive pattern, and a moving brush adapted to sequentially engage the teeth of the comb as the brush is displaced, is described. The position of the brush is deduced by starting from a known location and counting the pulses produced as the brush sweeps successive teeth of the comb. Such comb-shaped transducers require few input wires to the digital computer, but the position information provided thereby is volatile, i.e. the position information is lost if the computer is shut down and the sensor is not returned to its starting position.
Others have proposed to solve the transducer problem by the use of position encoders comprising a plurality of shaped conductive tracks and brushes, see for example FIG. 3a of U.S. Pat No. 4,051,489 issued Sept. 27, 1977 to Saito et al. The encoder-type transducers provide a non-volatile indication of position, but at the cost of many input leads to the microprocessor, one for each encoder track. If several encoders are employed in a camera to provide position information for several moving parts, the number of separate leads within the camera becomes considerable. All position transducers employing movable contacts share the problems of contact bounce producing spurious signals, and dirt or corrosion of the contacts causing noise. To overcome the corrosion problem, expensive gold-plated contacts have been used. Furthermore, the precision of the encoder-type position sensors is determined by the fineness of the encoder patterns, thereby increasing the cost and reducing the reliability of position transducers capable of highly accurate position measurements.
The challenge faced by the inventor therefore was to provide a position transducer for a digitally controlled photographic apparatus, such as a camera, that avoids the problems noted above. Required was a transducer that is inexpensive, relatively simple, low noise, nonvolatile, employing only a few leads for indicating a range of positions, and being capable of precision measurements which are not an inherent function of the complexity of the transducer.